The contamination of soil, concrete, aquifers and vegetation by ionic materials such as radionuclides and/or heavy metals has been a problem for decades. The danger, of course, is that humans and animals can be poisoned either directly or through the food chain. Our earlier-filed PCT application US90/03997, published on 7 Feb. 1991 as WO 91/01392 ("the '1392 application"), presented several new processes and apparatuses for decontaminating a bulk source such as soil or groundwater. These processes would also be useful for decontaminating concrete.
Inter alia, that application introduced the basic process in which an electropotential gradient is applied, via two or more electrodes, to a bulk source containing target ions. The electropotential drives the target ions into an ion permeable host receptor matrix located between the electrodes. Upon reaching the host receptor matrix, the ions are immobilized and/or confined within the matrix, thereby permitting removal of the ions along with the host receptor matrix.
Subsequent research by the present inventors identified additional methods which are especially useful for mobilizing ions such as radionuclides which tightly adsorb to soil clays. (As described below, soil clays can present a particularly tough impediment for decontamination.) Those methods are described in copending U.S. application Ser. No. 07/683,973, filed Apr. 8, 1991. That application describes methods which use wave energy (e.g., microwaves, radio waves, sonic and ultrasonic waves, etc.) to effect or enhance the dissociation of ionic species from a soil matrix.
It is against this background that the inventions described herein are brought, in an effort to improve upon, and refine, the processes and apparatuses described in those earlier applications.
The '1392 application provides an extensive history of prior attempts at soil and groundwater decontamination, which will not be repeated here. For convenience, however, a brief review of such prior attempts, as well as some of the problems which spurred the developments described herein, are discussed in the following sections.
A. SOIL DECONTAMINATION METHODS PA0 B. GROUNDWATER DECONTAMINATION PA0 C. DECONTAMINATING RADIOACTIVE SOILS AND CLAYS PA0 D. DECONTAMINATION USING ION EXCHANGE PA0 E. TECHNOLOGY OF THE '1392 APPLICATION PA0 F. DECONTAMINATION OF CONCRETE
Prior attempts to decontaminate soil have variously included: (1) excavating the contaminated soil and processing it to remove the target ions; (2) providing an impervious covering over the ion contaminated regions, which essentially immobilizes the ion by preventing the intrusion of the groundwater necessary for ion mobility; (3) vitrifying the entire soil strata in-situ by adding a "frit" to the soil and establishing an intense electrical field which turns the soil into a glass-like mass which prevents leaching of the ion; (4) injecting a polymerizable monomer or ion exchange gel into the soil; (5) washing the soil with surfactant chemicals and/or pH adjusters to remove soil contaminants, and returning the cleansed soil; (6) excavating contaminated soil for burial at a remote site; (7) burying the contamination by deep plowing using special machinery; (8) changing the land usage; and (10) thermally processing the soil, e.g., through calcination or incineration.
In Chemistry and Industry, 18 Sep. 1989, pp. 585-590, Lageman et al. describe another variation of the in-situ treatment which involves an electrokinetic process. This method utilizes electrodes imbedded in the soil. The electrodes are accompanied by a chemical solution circulation system which circulates the chemical solution needed to minimize the electrode effects. The solution provides a mechanism for transporting the extracted contaminants to a central processing station. The concern for migration of chemical solution into the soil or groundwater is not adequately addressed by this process.
The foregoing methods, however, are regarded as only marginally effective in achieving their goal, namely the return of contaminated land to its original use.
Decontaminating groundwater can be extremely difficult. In some cases of contaminated underground aquifers, remediation is so impractical that the only economical solution involves identifying and isolating the source of contamination, and then delaying human contact until natural diffusion of water through the aquifer can provide for dilution of the contaminant.
In other instances, groundwater is treated with conventional ion exchange media, and/or charcoal absorber beds. Yet another option involves in-situ treatment in which chemicals are introduced which react with the ionic contaminants to form insoluble precipitates.
In the past, cesium contaminated soils have posed a major obstacle to soil cleanup programs. The difficulty relates to the low mobility of heavy metals such as cesium and plutonium in soil. It has been shown that clays provide the repository for deposited cesium. For example, Gale et al. in 1964 showed that 70% of the total cesium was within the 13% clay fraction of a sandy loam. Lomenick and Tamira in 1965 made measurements of lake sediments and concluded that 84% of the cesium was associated with the 35% clay fraction. Also, it has been found that interleaved mica, which is a constituent of this sediment, was the receptor for the cesium.
The significance of the above conclusion becomes apparent when considering that the cage structure of muscovite mica, which is a component of soil clays, is very similar to that of the zeolite, chabazite. Soil clays are present in a percentage ranging from 10-25 percent in virtually every fertile soil in the world.
Heretofore, removing such heavy radioactive species from the soils has been so difficult as to be considered impracticable. The reasons for this difficulty lie in the atomic characteristics of these heavy metals, especially in their ionic state ("M+"). Generally, the larger the M+ ion, the more numerous are its insoluble salts. One of the important properties of heavy metal ions is their tendency to become bound in a zeolite in insoluble form.
This phenomenon involves a cage-like ion trap found in zeolites, which is responsible for the zeolites often being referred to as an ion sponge or ion exchanger. An example is the cesium ion (Cs+) which is routinely encountered in radioactive form when dealing with fission-type nuclear power or nuclear weapons. The crystal radius of this cesium ion is 3.4 angstroms and the hydrated radius is 6.6 angstroms. The hydrated radius being the radius the cluster that consists of the ion and the water molecules which surround it. The zeolite mineral of the chabazite class has receptor sites whose pore size ranges from 3-7 angstroms which is such that the cesium (without its hydration layer) will fit nearly perfectly into this "host-cage structure". By comparison, a smaller ion (sodium for example) would tend to be more weakly bound in the cage, and upon the arrival of a cesium ion, would be easily displaced.
Many of the currently employed methods for decontamination of water and soils involve some form of diffusion-controlled ion exchange.
For example, one of the world's largest plants for treatment of effluent from a spent nuclear fuel plant is the British Nuclear Fuels, Inc., SIXEP Plant. The SIXEP process is based on ion exchange using an inorganic ion exchanger. In this process, the positively charged cesium and strontium ions are taken up into the crystal lattice of clinoptilolite (a class of porous crystalline aluminosilicates of the zeolite family), in preference to the sodium ions which are naturally present. In this process, water containing the radioactive ions is caused to pass into close proximity to the clinoptilolite, whereupon diffusion takes place to cause migration of the radioactive ions into the clinoptilolite lattice, displacing the sodium ions.
In more recent technology, novel electrochemical cells incorporating ion exchange membranes have been used to rid water of metal ion contaminants in an economical manner. However, such cells generally have not been favored because the waste form (or concentrated effluent) is liquid, which is less desirable than solid waste.
The main disadvantage of the existing electrodialysis technology is that its use in an in-situ remediation or cleanup of either soil or groundwater is extremely limited, partly because of the presence of ionic colloids which will "blind" or plug the membrane, and partly due to the limitation in transport of a "complexed" ion species. Also, these cells can involve relatively complex operation, substantial capital investment in operating hardware, and a liquid waste form which in some cases can comprise a very large volume of waste. Most importantly, none of these processes are able to remove soluble ions from a bulk source.
The basic technology of the '1392 application represented a significant advance over prior decontamination techniques. Nevertheless, as is the case with most new technology (and most inventors), the desire for further refinement and improvement of their earlier technology accompanied the present inventors' subsequent research. Inter alia, two areas of interest prompted the further research leading to the inventions discussed herein and in the prior-filed application Ser. No. 07/822,959.
First, the present inventors realized that the '1392 application processes and systems were not achieving maximum efficiency. They discovered that inefficiency in the system could be due to the presence of unwanted hydrogen (H.sup.+) and/or hydroxyl (OH.sup.-) ions in the bulk source. Those ions, which are generated in large quantities at the anode and cathode, are relatively small and thus much more mobile than the larger ions which are the target of decontamination. Once formed, they can migrate rapidly, via the current generated by the electrodes, to the opposite electrode. Further, the generation of hydroxide ions at the cathode raises the pH in the vicinity of the cathode which causes precipitation of the contaminant cations as metal hydroxides. Thus, it was realized that much of the current being generated by the electrodes was being spent on moving the hydrogen and hydroxyl ions, and not the target ions. Further still, an efficient means of isolating the contaminants at the electrodes would be needed.
Secondly, the present inventors realized that treating large areas of contaminated land, such as the land surrounding Chernobyl, required a system having several properties. First, the system should be designed for use where most of the contamination is relatively near the surface. Second, the system should be designed to be able to cover large surface areas, and receive and confine large volumes of contaminants. Third, the system should be designed to be reusable and/or recyclable. Lastly, the system should be relatively mobile such that it readily can be moved to a new area when decontamination of the current area is completed.
As mentioned above, devices and methods for accomplishing the foregoing design goals are advanced in application Ser. No. 07/822,959.
The most common method for remediation of concrete contaminated with radionuclides, heavy metals, and organic compounds is mechanical scabbing followed by HEPA vacuum collection. Another developmental technique uses microwave energy to spall the concrete surface. Both of these methods produce large volumes of low-level radioactive waste (LLRW)--primarily contaminated rubble and secondary waste (fines, filtration media). They are costly, destructive (in the sense that they involve removal and disruption of the concrete itself), and create expensive waste disposal problems.
Slater et al. in their paper entitled, "Electrochemical Removal of Chloride from Bridge Decks" describe experiments attempting to devise a method of protecting bridge deck reinforcement bar or "rebar" from corrosion by extracting chlorides from the concrete using an electric field. Ponding frames are used to hold an ion exchange resin and an electrolyte solution on top of the bridge deck. The resin is slurried in the electrolyte, and an anode is positioned above the ponding frame, in contact with the electrolyte solution. A potential gradient is applied between the rebar and the electrode (anode) located above the bridge deck, and chloride ions are driven away from the rebar.
Vennesland et al., U.S. Pat. No. 4,832,803 discloses a method for removing chloride ions from steel reinforced concrete. This method uses graphite electrode nets as the anode and the rebar as the cathode. They use a viscous electrolyte, such as retarded gunite, a gel-like cement, that can adhere to vertical or down-facing surfaces. Once measurements of core samples show that a sufficient amount of chloride has been removed from the vicinity of the rebar, the gunite and graphite nets are removed and the surface of the decontaminated concrete is sandblasted. Repair concrete is then coated onto the decontaminated surface.
U.S. Pat. No. 5,141,607 to Swiat also discloses a method for electrochemically removing chlorides from steel reinforced concrete structures. In this method one electrode is placed in contact with the surface of the concrete and the steel reinforcing material (rebar) serves as the other electrode. The electrode and concrete are continuously saturated with an aqueous electrolyte, and the chloride is moved within the concrete away from the rebar.
There are a number of significant limitations to the methods and work disclosed in the above publications. For example, while adequate for the purposes described, (i.e., movement of a single, concentrated ionic species--chloride), the utility of these systems is severely limited by the following operational and design characteristics, especially when applied to full field scale scenarios.
Swiat relies on an aqueous saturation of the electrode-to-concrete interface, and of the pore structure within the concrete. While this condition is acceptable, and perhaps even enhances removal of the highly mobile chloride ion, the usefulness of this approach is limited when the target contaminant has a lower mobility, or is insoluble (e.g., when heavymetals, radionuclides, and organic compounds are to be removed). This limitation arises from the inability to control the competing effect of electroosmosis--the flow of water due to the disproportionate migration of a cation or anion in an electric field.
While Slater et al. do not require a saturation of the concrete per se, the effects of electroosmosis also are problematic with their method. The electrolyte seeps into the concrete, and there is a migration of ions towards the cathode. This flow of ions is against the direction of desired flow of chloride ions, and thus the effectiveness of this method to remove chloride ions from the vicinity of the rebar is limited. The negative effects of electroosmosis are manifested by the decrease in efficiency of the method that is observed over time.
Diffusion of electrolyte from the surface cell into the bulk concrete matrix has been shown by Slater et al. to be a concern. An excess of free, interstitial water in an electrical field creates a situation where electroosmosis becomes the dominant phenomenon. Electroosmosis in a concrete matrix predominates towards the cathode (rebar) because of the tendency of cations to migrate towards the cathode when the bulk medium has a fixed negative surface charge, such as is seen with concrete. Thus, the anionic electromigration is interfered with (if not reversed) by the competing migration due to electroosmosis. Because the majority of target contaminants are only soluble in alkaline conditions as anions, recovery of such anionic contaminant species is greatly limited by the effects of electroosmosis.
In addition to electroosmosis, diffusion of the electrolyte into the bulk source can cause other problems. Diffusion of the electrolyte can carry the contaminant back into the concrete, even as it is being cleaned. Depending on the porosity of the concrete matrix, even ion exchange media (i.e., beads) can be carried with the electrolyte as it diffuses. The ability to remove trace heavymetal and radionuclide contaminants under these conditions is very limited.
Furthermore, when electrolyte is lost to the bulk source, the benefits of its buffering capacity also are lost and a pH gradient is established across the concrete itself. This easily can trigger precipitation of the contaminant ion as it migrates towards the electrode, especially if the target ion is a heavy ion with marginal solubility.
Another problem associated with the method of the '607 patent is secondary contamination waste generated as a result of the constant circulation, or controlled flow, of aqueous electrolyte across the anode and through the various porous absorbants. In instances where the chloride ion is actually removed from the concrete (as opposed to merely being moved away from the zone immediately surrounding the steel rebar but not removed from the concrete per se), the electrolyte effluent contains the removed contaminant. This activity creates a secondary contamination waste stream which must be treated and disposed of. Such secondary waste is of a particular concern when the contaminants removed are radioactive, or otherwise hazardous.
While the viscosity of the electrolyte used by Vennesland et al. limits the negative effects of electrolyte diffusion, this method has other disadvantages and drawbacks which limit its effectiveness and usefulness. First, the method requires that the electrode be physically attached to the surface of the concrete. It is not a mobile electrode that can moved easily from area to area during the treatment process. After the chloride ions have been made to migrate away from the rebar, substantial post-treatment steps are required to return the concrete to its original condition. The electrolyte must be washed away, and the surface of the concrete must be sandblasted and repaired with repair concrete. These additional steps are laborious and time consuming, and would create secondary waste stream concerns if any chloride had reached the electrolyte or the surface of the concrete.
Further, while Vennesland et al. seek to provide a less expensive method of treatment than provided by Slater et al. by using lower voltages to induce chloride ion migration, since such lower voltages must be applied over a longer period of time, the total savings may be negligible.
In sum, the foregoing methods disclose moving highly mobile chloride ions away from the rebar. They do not disclose a method for efficiently removing the ions from the concrete, and in fact, Slater et al. recognize that their method is most effective in the area immediately surrounding the rebar, with chloride removal decreasing as distance from the rebar increases. While such results may be satisfactory when the goal of the forced migration of ions is to protect the rebar from corrosion by chloride ions, when circumstances require the removal of ions from concrete, as when radionuclides are to be removed, a mere rearrangement of ions within the concrete would not be acceptable. Furthermore, because a substantial amount of ions remain within the concrete, there is the possibility that the ions may migrate and be redistributed to the areas from where they were removed.
There is thus a need for a method of effectively cleaning concrete that is contaminated with marginally soluble, weakly ionic, bulky contaminant ions and ion complexes.
F. BIOREMEDIATION
The use of microbes and their byproducts, such as enzymes, to promote degradation or removal of toxic materials, a process commonly referred to as bioremediation, is a method being employed with increasing frequency at a number of contaminated sites. Some examples of this technique include: adding nutrients to contaminated zones to promote the growth of naturally-occurring microbes which are capable of degrading crude oil (such as that spilled from the Exxon Valdez) and the introduction into contaminated zones of microbes which are known to metabolize poisonous hydrocarbons, such as pesticides and other synthetic contaminants.
Some microbes are known to accumulate heavy metals, or change the properties (e.g., solubility) of heavy metals in situ, such that the metals are in a less dangerous form. This microbial activity is frequently referred to as biosorption. Desulfovibrio desulfuricans has been used to enhance uranium removal from solution via reduction from U(VI) to U(IV), Penicillium chrysogenum is known to uptake radium from soils, and Rhizopus arrhizus absorbs a variety of metals, such as zinc, copper, iron, and uranium.
However, the fact that these microbes have an ability to absorb contaminants does not mean that remediation of a site is easily accomplished. The success of this technology depends on the ability to direct the movement of microbes through the bulk medium to the contamination. Microbial movement is multidirectional, and can often be blocked by natural gradients, such as pH, moisture and nutritional changes, found in the bulk source.
A number of studies have been conducted by others to characterize methods of controlling microbial movement in bulk mediums such as aquifers and soils. One technique involves the use of an imposed electropotential gradient to "electrophorese" microbes through a bench-scale soil and water aquifer test bed.
As with other attempts to use induced electropotential gradients and bare electrodes for bulk media remediation, limitations of this process are observed. The most notable effect is the change in pH due to generation of H.sup.+ and OH at the electrodes. Because most microbes are extremely sensitive to changes in PH, the effects of this shift can range from reduced efficiency to death of the microbes.
Thus, there is a need for a process of enhancing bioremediation that allows the user to control the migration of the microbes so that a more thorough and efficient cleaning is performed.